My iMac is taking so long to boot up

Your iMac is a computer it is NOT a display for another device. You do NOT need to buy an Apple display, you can if you want of course however there are hundreds of external displays that will work with a MacBook Pro or MB Air. The ones I am leaning to are the Samsung displays.

Click https://www.samsung.com/us/monitors/ and then begin researching one that suits your needs and then begin shopping for it.

Your old 2017 likely either has a HD or a Fusion Drive, a Fusion Drive is a HD and a small SSD. The HD in any computer is the Achilles heal and the component most likely to fail. New Macs since 2021 have all come with SSD's which are considerably faster than any HD based system.

Unfortunately, there are very few choices in 27" 5K or 24" 4K displays.

You might want to look at 27" 4K (3840x2160) monitors.

Some of those that have IPS panels, near-100% coverage of sRGB, and modern inputs (DisplayPort, HDMI, and/or USB-C (DisplayPort)) can be found for as little as $300 – $350. If you run them in Retina "like 2560x1440" mode, they're not as sharp as a 5K Retina iMac or 5K Studio Display, but you're still probably looking at an improvement over an actual 27" 2560x1440 monitor.